1. Field of the Invention
The present invention relates to a filtering apparatus and method, and more particularly to a filtering apparatus and method in mobile communication apparatus.
2. Description of the Prior Art
To enhance the capability of data processing of mobile phone and base station, the researchers of wireless system develop the technology of 2.5 G for the operator of wireless communication to provide the users with voice and high-speed data service without replacing the whole telecommunication infrastructure.
An important technology, the Enhanced Data for GSM Evolution (EDGE), interests the researchers of 2.5G system. The EDGE uses the modulation method of improved 8PSK and is an innovative technology to the current GSM system. The EDGE may provide the data transmission rate of 384 kbps for system to supply the solution of voice, data, Internet linkage, and other kinds of mutual linkage. The GSM 5.04 provides the standard for the modulation method of 8PSK.
Please refer to the modulation method of 8PSK of EDGE stipulated in GSM 5.04 (V8.0.8). The modulation method of 8PSK comprises the three following steps: First, perform an input with a Gray mapping. Second, perform with a 3π/8 phase shift. Finally, get an output by a filtering process, and provide the output for a RF system to generate a corresponding RF signal.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram of converting a digital bit to a first vector by Gray mapping of the prior art. FIG. 2 is a schematic diagram of converting the first vector shown in FIG. 1 to a second vector by the 3π/8 phase shift module. The input of 8PSK system is a series of digital bits. A set of every 3 digital bits (d3i, d3i+1, d3i+2) is treated as one signal processing unit. The 8PSK system converts the set of every 3 digital bits to a first vector (Si=ej2πl/8) by Gray mapping. The first vector will be converted to a second vector (Ŝi=Si·eji3π/8) by 3π/8 phase shift. The second vector is output to be a modulation signal (Ii, Qi) by a filtering process. The modulation signal is the output of 8PSK system and is taken as a baseband signal. The baseband signal may be provided to a RF transmitter to proceed with the signal modulation. The object is to generate a RF signal for wireless communication. The modulation signal (Ii, Qi) is also called y(t′) and may be shown as the following equation:
      y    ⁡          (              t        ′            )        =            ∑      i        ⁢                            s          ^                i            ·                        c          o                ⁡                  (                                    t              ′                        -                          i              ⁢                                                          ⁢              T                        +                                          5                2                            ⁢              T                                )                    wherein T is a time period.
The above equation represents that y(t′) is the result of multiplying a plurality of second vectors with the corresponding predetermined distribution waveforms or called filtering coefficient Co(t) and summing up the values.